It was announced, in late 2005, that, having fulfilled contractual obligations to indie label, Relapse Records, Mastodon had been offered a deal by a major music label, a spot on the Warner Music roster alongside such bands as Green Day and My Chemical Romance. Despite this, however, Relapse Records released a DVD, and re-released the band's only demo.

An album titled Call of the Mastodon, comprising the material recorded for the band's first and only untitled demo, which featured Eric Saner on vocals, was released in early 2006. A DVD entitled "The Workhorse Chronicles" was released not long after and a full length album, the band's Warner Music debut, Blood Mountain, was released September 12th, 2006. In an interview with MTV news, Brann Dailor revealed that the band's sound, on this upcoming release would be decidedly different and more "progressive." He also expressed excitement over the possibility of the birth of a new "prog movement," naming bands such as Isis, along with Mastodon, as prospective leaders of said movement.

Blood Mountain met critical acclaim with reviews ranging from Metal Hammer and Kerrang! who stated it was as good or better than their last release Leviathan, while other critics view it as the essential release of the decade and UK Metal Hammer named it best album of '06. Blood Mountain also featured guest appearances by Scott Kelly of Neurosis on the track "Crystal Skull", Josh Homme of Queens of the Stone Age on "Colony of Birchmen", as well as The Mars Volta's keyboardist Isaiah "Ikey" Owens on "Pendulous Skin" and singer Cedric Bixler-Zavala on "Siberian Divide".

Mastodon's fourth album Crack the Skye was released on March 24, 2009 as a normal version and a deluxe version (which includes all songs in instrumental versions as well as their normal versions). The album is produced by Brendan O'Brien, and Scott Kelly of Neurosis returns as a guest musician on the title-track. While the lyrics have been reported to explore topics relating to Czarist Russia, the band has denied that the album is about Grigori Rasputin. These rumors started after a drawing of Rasputin was featured in merchandise. The band had debuted three new tracks at the Bonnaroo Music Festival, but did not play anymore live before the album's release, due to fears of internet distribution and wanting to play the songs to the right audience. In a MusicRadar interview, guitarist Bill Kelliher confirmed that the album is about an "out of body experience," and looks at the concepts of astral travel, wormholes, Stephen Hawking's theories and the spiritual realm. Drummer Brann Dailor has a section of vocals on the song "Oblivion" following the intro and chorus. The band has announced that they will play the album in its entirety incorporating a stage show on their upcoming tour. Mastodon was a headliner at the Scion Rock Fest on February 28, 2009, performing a set containing three tracks from Crack The Skye, the first time these songs were played since being finalized and recorded. Scott Kelly made an appearance performing his guest vocals for the songs "Crack the Skye," "Aqua Dementia," and "Crystal Skull."

Mastodon have previously appeared on the second stage at Ozzfest 2005 and have toured with the likes of Slayer, Lamb of God and Children of Bodom on the Unholy Alliance US tour and have 3 songs recorded live on the accompanying tour DVD.

Mastodon is:
Brann Dailor - Drums. (Formerly of Today Is the Day, Lethargy)
Troy Sanders - Vocals, Bass. (Also in Social Infestation, Four Hour Fogger)
Bill Kelliher - Guitar. (Formerly of Today Is the Day, Lethargy)
Brent Hinds - Guitar, Vocals. (Also in Fiend Without A Face)

Aforementioned former member, Eric Saner, left the band soon after the release of the 2000 demo. Troy Sanders, the then bassist, was announced as the new lead vocalist, although vocal duties were increasingly shared with guitarist Brent Hinds to the point where they are considered co-vocalists.

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metal, sludge, progressive metal, metalcore, sludge metal